1. Field of the Invention
The present invention generally relates to a language translation system. More particularly, the invention is concerned with a machine translation system for translating sentences described in a first language into sentences described in a second language, which translation system is preferably suited for translating with high efficiency a text which is composed of a plurality of components such as title (headline), summary, body sentences and others as in the case of articles found in periodicals, newspapers and the like.
2. Description of the Prior Art
At present, studies and development of machine- or computer-translation systems for translating Japanese into English sentences or reversely English into Japanese sentences are actively in progress. So far as the texts directed to the subject matters or objects belonging to a particular domain such as those of technical reports and manuals or the like are concerned, the machine translation technique has already reached such a level at which the sentences resulting from the machine translation can be meaningfully used although some correction through manual procedure by the user is often required after the translation.
The hitherto known translation systems can be classified into four categories mentioned below in dependence on the domains of the objects for translation.
1. A so-called electronic-dictionary based translation in which the translation is performed with reference to a dictionary on the word basis.
2. A translation which is performed on the basis of template sentences stored in the form of sentences. The translation performed by replacing some words of the template sentence by other word(s) falls within this second category.
3. A translation relying on the parsing of a complicated sentence of given patterns.
4. A translation directed not only to the sentence but also to a text as a whole (i.e. a meaningful set of sentences). The translation technique of this category is now in the course of being developed as a part of artificial intelligence (AI) systems for making it possible to analyze and understand the context among the sentences and discourse. Assuming, by way of example, that a sentence "He . . . " is given, the translation of this fourth category is to be performed in such a manner in which who is indicated by "He" can be explicitly understood by parsing the context relative to the preceding sentences.
On the other hand, in conjunction with the method of analyzing a sentence or a set of correlated sentences (commonly referred to as the parsing of sentence or sentences), descriptive methods standardized as far as possible have been adopted for the purpose of facilitating the handling of grammar rules. By way of example, rewriting rules that a string of "article noun" be handled as a "noun phrase" and so forth are adopted to assemble the components of sentence. When the resulting "sentence" has proved to be meaningful, it is decided that the parsing has been carried out successfully and correctly. This type of parsing based on the rewriting rules is commonly adopted. Further, there have been known a method according to which the rules applied to the endings of words for conjugation thereof are handled separately and distinctively from the rules to be applied to the words and phrases in such a manner that "noun-phrase verb-phrase" is considered to be "a sentence", and a method according to which different groups of rules are applied in the flow of translation processing such that a group of rules are applied to the parsing of input language while another group of different rules is employed in creation of the output language.
Besides, such a viewpoint that the grammar should be considered to differ in dependence on the domains of the objects for translation is admitted by the grammarians (Reference may be made, for example, to R. Quirk et al "A Grammar of Contemporary English, Section 7, 90, Block Language" Longman, 1972).
Additionally, such a method may be adopted in which the ambiguities encountered in the parsing are eliminated by utilizing the section numbers and indexes as clues for the purpose of disambiguation in addition to the grammar rules (although some problem will arise with respect to the manner in which the section numbers and the indexes are handled). As an example of such method, there may be mentioned the one disclosed in Hirose et al's article entitled "Source Form Analysis in English-Japanese Machine Translation System" contained in the collection of lectures at the 28-th National Conference of the Information Processing Society of Japan, pp. 903-904.
With the above methods, however, the user view point is not yet fully satisfied as to what function is to be seriously considered in translation.
There is also known a method as disclosed in Japanese Patent Unexamined Publication No. 59-223883 which has been proposed to meet the requirements of high quality translation to analyze and understand a profound meaning of sentence(s). According to this method, the domain of the object for translation as well as the sentences thereof are narrowly limited to the extent that there occurs no ambiguity of sentence structure and meaning, or in other words to the extent that the gaps among template sentences are filled, to thereby realize high quality translation. This method is hard to process a text belonging to a domain even with a slight difference from the narrowed domain so that it can hardly be applied to a translation system which cannot limit the domain of a source text. Further, since there is almost no ambiguity in the parsing if the meaning is narrowly limited, the text translated using only a simple rewriting rule (e.g., a rule of handling a string of "article+ adjective+noun" as a "noun phrase") can be used as a meaningful draft, without necessity of a meaning processing.
On the other hand, in case where a free translation is desired or an answer is requested for an inquiry, an expression particularly defining the meaning to some degree becomes necessary. This results in a very large number of meaning domain models while necessiating to eliminate the ambiguities among those meanings. However, some users may desire to take prominence to the speed of translation rather than eliminating meaning ambiguities. Conventional methods have not considered such case.